Definitions
"Scanning probe microscope" (SPM) means an instrument which provides a microscopic analysis of the topographical features or other characteristics of a surface by causing a probe to scan the surface. It refers to a class of instruments which employ a technique of mapping the spatial distribution of a surface property, by localizing the influence of the property to a small probe. The probe moves relative to the sample and measures the change in the property or follows constant contours of the property. Depending on the type of SPM, the probe either contacts or rides slightly (up to a few hundred Angstroms) above the surface to be analyzed. Scanning probe microscopes include devices such as scanning force microscopes (SFMs), scanning tunneling microscopes (STMs), scanning acoustic microscopes, scanning capacitance microscopes, magnetic force microscopes, scanning thermal microscopes, scanning optical microscopes, and scanning ion-conductive microscopes.
"Probe" means the element of an SPM which rides on or over the surface of the sample and acts as the sensing point for surface interactions. In an SFM the probe includes a flexible cantilever and a microscopic tip which projects from an end of the cantilever. In an STM the probe includes a sharp metallic tip which is capable of sustaining a tunneling current with the surface of the sample. This current can be measured and maintained by means of sensitive actuators and amplifying electronics. In a combined SFM/STM the probe includes a cantilever and tip which are conductive, and the cantilever deflection and the tunneling current are measured simultaneously,
"Cantilever" means the portion of the probe of an SFM which deflects slightly in response to forces acting on the tip, allowing a deflection sensor to generate an error signal as the probe scans the surface of the sample.
"Tip" in an SFM means the microscopic projection from one end of the cantilever which rides on or slightly above the surface of the sample. In an STM, "tip" refers to the metallic tip.
"Package" means an assembly which includes the cantilever and tip, a chip from which the cantilever projects, and may include a plate on which the chip is mounted.
"Scanning Force Microscope" SFM (sometimes referred to as Atomic Force Microscope) means an SPM which senses the topography of a surface by detecting the deflection of a cantilever as the sample-is scanned. An SFM may operate in a contacting mode, in which the tip of the probe is in contact with the sample surface, or a non-contacting mode, in which the tip is maintained at a spacing of about 50 .ANG. or greater above the sample surface. The cantilever deflects in response to electrostatic, magnetic, van der Waals or other forces between the tip and surface. In these cases, the deflection of the cantilever from which the tip projects is measured.
"Scanning Tunneling Microscope" (STM) means an SPM in which a tunneling current flows between the probe and the sample surface, from which it is separated by approximately 1-10 .ANG.. The magnitude of the tunneling current is highly sensitive to changes in the spacing between the probe and sample. STMs are normally operated in a constant current mode, wherein changes in the tunneling current are detected as an error signal. A feedback loop uses this signal to send a correction signal to a transducer element to adjust the spacing between the probe and sample and thereby maintain a constant tunneling current. An STM may also be operated in a constant height mode, wherein the probe is maintained at a constant height so that the probe-sample gap is not controlled, and variations in the tunneling current are detected.
"Kinematic mounting" means a technique of removably mounting a rigid object relative to another rigid object so as to yield a very accurate, reproducible positioning of the objects with respect to each other. The position of the first object is defined by six points of contact on the second. These six points must not over or under constrain the position of the first object. In one common form of kinematic mounting, three balls on the first object contact a conical depression, a slot (or groove) and a flat contact zone, respectively, on the second object. Alternatively, the three balls fit snugly within three slots formed at 120.degree. angles to one another on the second object. The foregoing are only examples; numerous other kinematic mounting arrangements are possible. According to the principles of kinematic mounting, which are well known in the mechanical arts, six points of contact between the two objects are required to establish a kinematic mounting arrangement. For example, in the first illustration given above, the first ball makes contact at three points on the conical surface (because of inherent surface imperfections, a continuous contact around the cone will not occur), two points in the slot, and one point on the flat surface, giving it a total of six contact points. In the second illustration, each ball contacts points on either side of the slot into which it fits.
The Prior Art
Scanning probe microscopes (SPMs) are used to obtain extremely detailed analyses of the topographical or other features of a surface, with sensitivities extending down to the scale of individual atoms and molecules. Several components are common to practically all scanning probe microscopes. The essential component of the microscope is a tiny probe positioned in very close proximity to a sample surface and providing a measurement of its topography or some other physical parameter, with a resolution that is determined primarily by the shape of the tip and its proximity to the surface. In a scanning force microscope (SFM), the probe includes a tip which projects from the end of a cantilever. Typically, the tip is very sharp to achieve maximum lateral resolution by confining the force interaction to the end of the tip. A deflection sensor detects the deflection of the cantilever and generates a deflection signal, which is then compared with-a desired or reference deflection signal. The reference signal is then subtracted from the deflection signal to obtain an error signal, which is delivered to a controller. There are several types of deflection sensors. One type uses an optical interferometer as described in an article by D. Rugar et al., Review of Scientific Instruments, Vol. 59, p. 2337 (1988). Most commercial SFMs, however, employ a laser beam which is reflected from the back of the cantilever and use a photodetector to sense the angular movement of the beam as the cantilever is deflected. The probe (cantilever and tip) and deflection sensor are normally housed in a unit referred to as a head, which also contains circuitry for preamplifying the signals generated by the deflection sensor before they are passed to a controller. An image is formed by scanning the sample with respect to the probe in a raster pattern, recording data at successive points in the scan, and displaying the data on a video display. The development of scanning (or atomic) force microscopy is described in articles by G. Binnig et al., Europhys. Lett., Vol. 3, p. 1281 (1987), and T. R. Albrecht et al., J. Vac. Sci. Technology, A6, p. 271 (1988). The development of the cantilever for SFMs is described in an article by T. R. Albrecht et al., entitled "Microfabricated Cantilever Stylus for Atomic Force Microscopy". J.Vac. Sci. Technol., A8, p. 3386 (1990). Other types of SPMs, such as scanning capacitance or scanning magnetic force microscopes, also use similar deflection sensors.
A scanning tunneling microscope (STM) is similar to an SFM in overall structure, but the probe consists of a sharpened conductive needle-like tip rather than a cantilever. The surface to be mapped must generally be conductive or semiconductive. The metallic needle is typically is positioned a few Angstroms above the surface. When a bias voltage is applied between the tip and the sample, a tunneling current flows between the tip and the surface. The tunneling current is exponentially sensitive to the spacing between the tip and the surface and thus provides a representation of the spacing. The variations in the tunneling current in an STM are therefore analogous to the deflection of the cantilever in an SFM. The head contains circuitry for biasing the tip with respect to the sample and preamplifying the tunneling current before it is passed to a controller.
Before a desired region on the sample can be analyzed in an SFM, it must be positioned properly with respect to the probe, that is, the probe must be positioned above the location on the sample to be examined and must be brought into contact or close proximity with the sample. This requires two types of movement: first a lateral (x,y) movement and then a vertical (z) movement. The translations required to do this are beyond the limited range of the x,y,z fine movement stage. This process may be accomplished manually or with "coarse" positioning stages. In the latter case, the sample or head is mounted on a coarse x,y stage, which is capable of horizontal movement in any direction to properly position the sample beneath the probe. Typically, a coarse x,y stage has a translation range of around 25 mm.
A coarse z stage is used to position the probe vertically with respect to the sample. It is desirable that a z coarse stage permit maximum sample-probe separation (e.g., 30 mm or more if possible). In this position, the probe can be changed if necessary and/or a different sample may be placed in the SPM. The coarse z stage is also adjustable to bring the probe to a distance (e.g., of around 100 .mu.m) where the position relationship between the probe and sample can be viewed through an accessory optical microscope. The coarse x,y stage is then used to move the sample horizontally with respect to the probe until the optical microscopic view indicates that the probe is positioned over a feature or area of the sample which is to be analyzed. The coarse z stage is then adjusted carefully so as to bring the probe to the sample gradually until the scanner fine x,y,z stage (scanner, described below) and its associated feedback loop (described below) take over to maintain a proper probe-sample separation. The final approach requires a resolution of about one micron and must be performed delicately to avoid crashing the probe into the sample.
In all of the coarse and fine (scanning) movements, the key factor is the position and movement of the probe relative to the sample. The actual movement may be performed by the probe or the sample or both.
The scanning operation is performed by a fine x,y,z stage, or scanner, which has a range of about 1-300 .mu.m in the x and y directions and about 1-15 .mu.m in the z direction. The scanner typically moves the sample horizontally such that the probe follows a raster-type path over the surface to be analyzed. In the fast scan direction, a computer collects a line of data at a series the scanner for the next line of data points to be taken. The resulting image will be made up of individual pixels. Usually, all data are collected in the same fast scan direction, that is, data are not collected along the reverse path.
In most SPMs, the scanning movement is generated with a vertically-oriented piezoelectric tube. The base of the tube is fixed, while the other end, which may be connected to either the probe or the sample, is free to move laterally as an input voltage signal is applied to the piezoelectric tube. The use of a piezoelectric tube in this application is well known and is described, for example, in an article by Binnig and Smith, Review of Scientific Instruments, Vol. 57, pp. 1688 (August 1986).
Fine movement in the z direction is normally also obtained using a piezoelectric device. FIG. 11A illustrates the prior art feedback loop for controlling the movement of the scanner in the z direction. Assuming the device is an SFM or other device that uses a similar type of cantilever, a deflection sensor measures the deflection of the probe and generates an error signal E which is the difference between the deflection signal and a reference signal. The error signal E is passed to a controller which applies a z feedback voltage signal Z.sub.v which drives a scanner in the z direction so as to maintain a constant cantilever deflection as the sample is scanned horizontally. For example, if the probe encounters a bump in the sample surface, the feedback signal Z.sub.v will cause the scanner to increase the separation between the probe and the sample and thereby maintain a constant cantilever deflection. The feedback signal Z.sub.v thus represents the sample topography and can be used to form an image. Alternatively, the SFM may be operated with the feedback adjusted so as to compensate only for large topographical features such as sample slope, and the error signal E may be used to generate a representation of the sample surface. This mode has disadvantages. For example, damage to the surface or probe may occur if the probe deflection exceeds a maximum limit.
In the prior art the function-of the controller may be achieved purely by analog circuitry, in which the error signal is appropriately processed in order to optimize the performance of the feedback loop. Alternatively, the error signal may be digitized, and the processing may be performed digitally using a computer or digital signal processing device, such as are commonly known and available. In the latter case, the digital signals are converted back into analog form before they are transmitted to the scanner.
The feedback loop in an STM operates in a very similar manner, the primary difference being that the error signal which is sent to the controller is generated by the tunneling current rather than the deflection of a cantilever. The difference of this current from a set value, which is a function of the spacing between the probe and the surface, is used by the controller to determine the z feedback signal which it sends to the scanner. The feedback signal adjusts the scanner position to maintain constant spacing between the probe and the surface. Since the tunneling current depends exponentially on the spacing between the probe and the surface, a high vertical sensitivity is obtained. Because the probe may be atomically sharp, the lateral sensitivity is also high.
The topography of the sample is often displayed in a format known as a grey scale, in which the image brightness at each pixel point is some function of the surface height at that point on the surface. For example, when the z feedback signal applied to the scanner causes it to pull the sample back (e.g., to compensate for the height of a peak on the surface) the corresponding data point on the display is painted bright Conversely, when the sample is moved towards the probe (e.g., to compensate for the depth of a valley) the data point is painted dark. Each pixel on the display thus represents an x,y position on the sample and the z coordinate is represented by intensity. The z position can also be represented numerically or graphically with high precision.
As stated above, it is known to measure the deflection of the cantilever in an SFM by directing a laser beam against a smooth surface on the back of the cantilever and detecting changes in the position of the reflected laser beam as the cantilever is deflected. The shift in the laser beam position is normally detected by a bi-cell position-sensitive photodetector (PSPD). With conventional SFM's, this detection circuitry generally obstructs an optical view of the probe positioned over the sample. Application Ser. No. 07/668,886, filed Mar. 13, 1991, which is incorporated herein by reference, describes a deflection sensor in which the laser beam is reflected from a mirror positioned to a side of the cantilever so that the view from directly above the cantilever is not obstructed. That application also describes a system for kinematically mounting the mirror in the deflection sensor and a mechanism for kinematically mounting the head on the base.
These represent significant improvements over the prior art. However, a number of difficulties remain with prior art scanning probe microscopes, including the following:
1. In an SFM, the probe normally wears out and must be replaced after several samples have been scanned. Moreover, it is often desirable to change probes between samples to avoid contaminating the surface of a new sample with material accumulated on the tip from a previous sample surface. With the type of deflection sensor described above, the laser beam must be precisely directed to a very small area, on the order of 20 microns wide, on the back of the cantilever. Each time the cantilever is replaced, the laser beam must be readjusted so that it strikes the same position. Aligning the deflection sensor is a time-consuming procedure and typically requires a very precise position stage. For example, scanning a sample might take 30 minutes, and repositioning the laser spot might take an additional 15 minutes. Thus, a large portion of the time spent on a sample must be used to realign the deflection sensor after the probe has been replaced.
2. Different preamplification circuitry is required to amplify either the signal from the deflection sensor in an SFM or the tunneling currents from the tip in an STM. These preamps must be located in the head, close to the source of their respective signals, to reduce noise pickup. Likewise, SFMs and STMs typically require different probes, also located in the head. In the prior art arrangements, a head is dedicated either to an SFM or to an STM. Consequently, the head must be disengaged and replaced in order to switch between SFM and STM operating modes. This is a time consuming procedure. Moreover, each head is an expensive component.
3. A bi-cell PSPD is typically used in the deflection sensor of an SFM to detect changes in light position caused by cantilever deflection. The sensitivity of bi-cells to these changes depends nonlinearly on the initial light position. Sensitivity is greatest when the light strikes the center of the PSPD, thereby producing a zero initial signal. As the light position moves off-center (i.e., an initial signal offset is present), sensitivity drops. If the initial offset is too large, the bi-cell cannot function, since light strikes only one of the cells. This nonlinear position response is further adversely affected by intensity variations across the width of the light spot. To minimize these effects, frequent and time-consuming adjustments to zero the initial signal offset are necessary before running the microscope and each time a probe is changed.
4. The coarse x,y stage in an SPM is often a stacked structure which has at least three levels: a fixed base, a y stage, and an x stage. This configuration has a relatively large mechanical loop, i.e., thermal and mechanical displacements in these individual stages are cumulative and can affect the spacing between the probe and sample. These displacements are a significant source of noise in the data. A configuration with a large mechanical loop may also be unstable.
5. Piezoelectric scanners inherently exhibit nonlinear behavior which includes hysteresis (where the scanner position for a given control voltage is a function of past history of movement), creep (where the scanner position gradually drifts in response to an applied voltage), and nonlinear response (where the scanner position is a nonlinear function of applied voltage). In addition, bending of a piezoelectric tube scanner is inherently associated with its lateral movement and causes it to tilt. These nonlinear effects contribute undesirably to the data image and require some means for scan correction. U.S. Pat. No. 5,051,646 describes a method to correct for these nonlinearities by applying a nonlinear control voltage to the piezoelectric scanner. However, this method is "open loop" i.e., it does not use feedback and has no means to determine and correct the actual scanner motion due to the applied nonlinear input signal. Application Ser. No. 07/766,656, filed Sep. 26, 1991, which is incorporated herein by reference, describes a method of correcting for nonlinearities in the x,y lateral motion of the scanner that is "closed loop" i.e. it does use feedback However, the method does not take into account the bending of a piezoelectric tube scanner, which causes tilt.
6. In typical SPMs the problem of hysteresis requires that each line of data in the raster scan be collected in the same direction, since data collected in the reverse direction includes the effects of hysteresis. As a consequence, each line of the raster scan must be traversed twice--once to collect data and once to return along the same path (or vice versa). The length of time necessary to generate an image is thus significantly greater than what it might be without hysteresis effects. Moreover, hysteresis problems prevent the use of data collected in the forward scan of a line to adjust the scan parameters before generating an image from scanning the line in the reverse direction.
7. Another source of error in the data image arises due to the thickness of the sample. As a piezoelectric tube scanner bends to thereby produce a lateral motion of a sample (or probe) mounted on it, the sample (or probe) moves in an arc-shaped path As the thickness (vertical dimension) of the sample increases, a given input signal to the piezoelectric tube scanner therefore produces a larger horizontal translation of the surface of the sample.
8. In order to position the sample relative to the probe, it is useful to have both a coaxial (on-axis) and oblique view of the same using an optical microscope.
These views provide means to monitor fine positioning of the sample relative to the probe. The coaxial view assists in positioning the probe over the feature of the sample to be measured. The oblique view permits accurate adjustment of probe orientation (for instance, cantilever tilt) relative to the sample surface. Conventional SPMs provide both these features; however, they are provided in two separate, manually operated microscopes, which are unwieldy to use. Obtaining these dual views is thus inconvenient.
9. In prior art SPMs the piezoelectric tube scanner cannot be operated at a rate greater than its resonant frequency. Above its resonant frequency, the response of the scanner to an input voltage signal is greatly reduced and out of phase with the input signal.
10. Prior art SPMs do not permit the adjustment of scanning parameters such as scanning rate or probe path in response to topographical features encountered by the probe.